Modern transmitters and receivers are often required to be able to operate in different frequency bands. This can be done using different solutions.
A first possibility is to use parallel transmitters or receivers where each transmitter/receiver is used for a specific frequency band. This is a costly solution in terms of component count, area and power consumption.
Another possibility is to use tunable elements in the transmitter or receiver, the elements being tuned for specific frequency bands. The tunable elements can use e.g. varactors, such that the transmitter or receiver can be tuned for operation in different frequency bands as it is shown by Chong-Ru Wu et al., “A 3-5 GHz Frequency-Tunable Receiver Frontend for Multiband Applications”, in IEEE MICROWAVE AND WIRELESS COMPONENTS LETTERS, VOL. 18, NO. 9, Sep. 2008. This solution requires expensive tunable components and additional digital circuitry for tuning the circuits to the correct frequency.
Yet another alternative, which is often used to support multiband operation is to switch between different frequency bands as it is shown by Atsushi Fukuda et al. “A High Power and Highly Efficient Multi-band Power Amplifier for Mobile Terminals” in 2010 IEEE Radio and Wireless Symposium conference publications pages 45-48. This can be done with capacitor banks and switches, or even by switching between complete matching networks. Using this alternative, additional circuitry is necessary for implementing the multiband operation.
In another alternative, one may use wideband circuits in the transmitter or receiver. However, the maximum bandwidth of practical building blocks is naturally limited. Apart from this, for a wideband receiver there is the disadvantage of being sensitive to unwanted sources that fall within the receiver bandwidth. In a similar way, a wideband transmitter will amplify unwanted input signals that fall within the transmitter bandwidth.
All the previously presented alternative solutions, except the last one, have in common that the transmitter or receiver are capable of transmitting or receiving in only one frequency band at a time, reducing the throughput of the system as it is shown by Jussi Ryynanen et al., “Integrated Circuits for Multi-Band Multi-Mode Receivers” in the Special Issue on Wireless Reconfigurable Terminals—Part II, IEEE Circuits and Systems Magazine, second quarter 2006.
Other alternative solutions are known from e.g. U.S. Pat. No. 6,658,265 and U.S. Pat. No. 6,917,815.
U.S. Pat. No. 6,658,265 discloses a dual mode amplifier capable of operating in a common mode for one frequency band and a differential mode for a second frequency band. In the common mode, the amplifier provides two identical signals to a matching network, and in the differential mode, the amplifier provides two signals that are 180° out of phase from one another to the matching network. The matching network is configured to maintain the same input and output impedance regardless of whether the amplifier is operating in the common mode or differential mode. Since the matching network operates on two signals, either common or differential, a power combining network is typically required to combine the two signals into a single signal for transmission. It is observed that the system does not support concurrent multiband operation.
U.S. Pat. No. 6,917,815 discloses an architecture for a concurrent dual band high-frequency receiver. It combines a concurrent dual-band front-end subsystem having a dual-band antenna, dual band pre-amplifier filter and concurrent dual-band LNA with an image rejection down converter to provide the functions of a typical receiver, including reception, amplification and down conversion of a signal in two discrete desired frequency bands simultaneously. The system disclosed here is a concurrent dual band receiver architecture but it doesn't make use of simultaneous common mode and differential mode operation.
It is therefore a need to obtain a front-end that works concurrently and provides both differential and common mode of operation with the advantage of reducing the costs and the footprint of the circuits.